While the electronics industry consumes a very large quantity of nitrogen gas, stringent requirements have been imposed on the purity of the nitrogen gas they use from the standpoint of maintenance of the high precision of parts. Nitrogen gas is generally produced from air in a production sequence which consists of compressing the air with a compressor, passing the compressed air through an adsorbent column to remove carbon dioxide gas and water, feeding the emerging air further to a heat exchanger where it is chilled by heat exchange with a refrigerant, feeding the chilled air to a distillation column for cryogenic liquefaction and separation to give product nitrogen gas, and finally passing the same through said heat exchanger to heat it up to a temperature near atmospheric temperature. However, the product nitrogen gas thus produced contains oxygen as an impurity and the use of this nitrogen gas as it is presents various prcblems. One of the methods for removing impurity oxygen (1) comprises adding a small amount of hydrogen to nitrogen gas and reacting the hydrogen in the mixture with the impurity oxygen in the nitrogen gas in the presence of a platinum catalyst at a temperature of about 200.degree. C. to remove the impurity oxygen in the form of water. Another method (2) comprises contacting nitrogen gas with a nickel catalyst at a temperature of about 200.degree. C. to remove the impurity oxygen by way of the reaction Ni +1/2O.sub.2 .fwdarw.NiO. However, as both methods involve the step of heating nitrogen gas to a high temperature for catalytic reaction, the corresponding hardware cannot be built into the nitrogen gas production line which is a cryogenic system. That is to say, the purification equipment must be installed independently of the nitrogen gas production equipment and this entails, of necessity, the disadvantage that the overall size of the production plant is increased. Furthermore, the first-mentioned method (1) requires exact control over the addition level of hydrogen. Unless hydrogen is added in an amount exactly commensurate with the amount of impurity oxygen present, either some oxygen remains in the product gas or the very hydrogen so added becomes a new impurity, so that high skill is required in operation. In the second-mentioned method (2), the NiO produced in the reaction with impurity oxygen must be regenerated (NiO+H.sub.2 .fwdarw.Ni+H.sub.2 O) and the cost of the H.sub.2 gas equipment for catalyst regeneration contributes to an increased purification cost. Solutions to these problems have been awaited.
Furthermore, the conventional nitrogen gas production equipment employs an expansion turbine for chilling the refrigerant used for heat exchange with compressed air from the compressor and this turbine is driven by the pressure of the gas generated by gasification of the liquid air collecting in the distillation column (as the result of cryogenic liquefaction and separation, the low-boiling nitrogen leaves the column, while the balance in the form of an oxygen-rich liquid air collects in the column). However, the expansion turbine has a high rotational speed (in the order of tens of thausands of revolutions per minute) and cannot easily follow a variation in load, thus requiring a specially trained operator. Moreover, as a high-speed machine, the expansion turbine not only demands high-precision in construction and is costly but requires specially trained personnel for its operation. These problems emanate all from the high-speed rotary mechanism of the expansion turbine and there has been a strong demand for elimination of the expansion turbine having such a high-speed rotary mechanism.